1. Field of the Invention
The present invention relates to a device that performs combining of light and mode conversion simultaneously for a planar optical waveguide device and a DP-QPSK modulator used in optical fiber communication.
Priority is claimed on Japanese Patent Application No. 2013-214792, filed on Oct. 15, 2013, the content of which is incorporated herein by reference.
2. Description of Related Art
Currently, the amount of information transmitted in optical communication has been increasing. In order to respond to such an increase in the amount of information, measures have been taken to increase the signal speed, an increase in the number of channels due to wavelength multiplexing communication, and the like. In particular, in digital coherent transmission technology of the next generation 100 Gbps (gigabit per second) for high-speed information communication, in order to double the amount of information per unit time, a polarization multiplexing scheme for carrying information in two polarized waves having electric fields perpendicular to each other is used. However, in modulation schemes for high-speed communication including the polarization multiplexing scheme, the structure of optical circuit components that form an optical modulator is complicated. For this reason, problems, such as a device size increase and a cost increase, occur. In order to solve such problems, an optical modulator having a planar optical waveguide using silicon, which is advantageous in terms of easy processing, size reduction by integration, and cost reduction by mass production, has been studied.
However, the polarization multiplexing in the planar optical waveguide has the following problems. In general, the planar optical waveguide has a shape in which the width direction parallel to the substrate and the height direction perpendicular to the substrate are asymmetric. For this reason, in two types of polarization modes of a mode in which an electric field component in the width direction is a main component (hereinafter, referred to as a TE mode) and a mode in which an electric field component in the height direction is a main component (hereinafter, referred to as a TM mode), the characteristics, such as an effective refractive index, are different. Among these modes, a fundamental TE mode (TE0) and a fundamental TM mode (TM0) are used in many cases. Here, TE0 refers to a mode having the largest effective refractive index of the TE modes. In addition, TM0 refers to a mode having the largest effective refractive index of the TM modes. It is difficult to perform an optical modulation operation for these modes having different characteristics with a single planar optical waveguide device. Accordingly, planar optical waveguide devices optimized for the modes are required. However, this requires a lot of effort in terms of the development of the planar optical waveguide devices.
As a method for solving this problem, a method can be mentioned in which TE0 is used as light incident on the planar optical waveguide device optimized for the desired TE0 and the output is polarization-converted to TM0. The polarization conversion herein indicates a conversion from TE0 to TM0. In order to perform the operation described above, a planar optical waveguide device for performing polarization conversion on the substrate is required.
As a technique for performing such polarization conversion on the substrate, an optical waveguide device has been proposed in which TE0 is converted to TE1 in a combining portion that outputs one high-order TE mode (TE1) for the input of one or two TE0 (hereinafter, an element including such a portion is referred to as a high-order mode conversion combining element) and then the TE1 is converted to TM0.
Here, TE1 indicates a TE mode having the second largest effective refractive index. As the related art, Non-patent document 1 (K. Mertens, et al., “First Realized Polarization Converter Based on Hybrid Supermodes”, IEEE Photonics technology letters, Vol. 10, No. 3 (1998)), Non-patent document 2 (Yunhong Ding, et al., “Wide-band Polarization Splitter and Rotator with Large Fabrication Tolerance and Simple Fabrication Process”, OFC/NFOEC Technical Digest, OTh4I.2 (2013)), and Non-patent document 3 (Yuji Matsuura, et al., “Low Loss of New Y-branch Structure Element”, 1994 Spring Meeting of Institute of Electronics, Information and Communication Engineers of Japan, C-330, April, 1994) can be mentioned.
As shown in FIG. 2A, the devices disclosed in Non-patent document 1 and Non-patent document 2 have a structure in which TM0 is used as an input, TM0 is input to a high-order polarization conversion element 21 that converts TM0 to TE1, and a branch portion 22 that divides TE1 into two TE0 is provided after the high-order polarization conversion element 21 (refer to FIG. 4(b) in Non-patent document 1 and FIGS. 1A to 1C in Non-patent document 2. Attention is paid to the reverse process in the invention. In this case, as shown in FIG. 2B, the structure in the related art is the same as a structure in which TE0 input to one or both of two input ports of a combining portion 22 is output as TE1 (that is, a high-order mode conversion combining element) and then TE1 is converted to TM0 after passing through the high-order polarization conversion element 21 (here, the branch portion is referred to as a combining portion since attention is paid to the reverse process; the same hereinbelow). That is, by using these elements, it is possible to perform polarization conversion and combining simultaneously.
In the devices disclosed in Non-patent document 1 and Non-patent document 2, it is thought that a Y-branch element is used as a high-order mode conversion combining element. This is determined from the diagrams or the waveguide size in the documents, even though Y-branch is not mentioned in the Non-patent document 1 and the Non-patent document 2.
A Y-branch element having a reduced manufacturing error by providing a gap between input portions is disclosed in Non-patent document 3. However, both an input portion and an output portion are single-mode waveguides through which only TE0 passes, and conversion to TE1 is not intended originally in the Y-branch element.